linux-kernel - Re: [PATCH 3/5] writeback: prevent sync livelock with the sync

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Message-ID: <20100729150241.GC12690@quack.suse.cz>
Date:	Thu, 29 Jul 2010 17:02:41 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>, Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 3/5] writeback: prevent sync livelock with the
 sync_after timestamp

  Hi Fengguang,

On Thu 29-07-10 19:51:45, Wu Fengguang wrote:
> The start time in writeback_inodes_wb() is not very useful because it
> slips at each invocation time. Preferrably one _constant_ time shall be
> used at the beginning to cover the whole sync() work.
> 
> The newly dirtied inodes are now guarded at the queue_io() time instead
> of the b_io walk time. This is more natural: non-empty b_io/b_more_io
> means "more work pending".
> 
> The timestamp is now grabbed the sync work submission time, and may be
> further optimized to the initial sync() call time.
  The patch seems to have some issues...

> +	if (wbc->for_sync) {
  For example this is never set. You only set wb->for_sync.

> +		expire_interval = 1;
> +		older_than_this = wbc->sync_after;
  And sync_after is never set either???

> -	if (!(wbc->for_kupdate || wbc->for_background) || list_empty(&wb->b_io))
> +	if (list_empty(&wb->b_io))
>  		queue_io(wb, wbc);
  And what is the purpose of this? It looks as an unrelated change to me.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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